In recent years, there has been remarkable progress in regenerative medicine (medical practice). Since a method of establishing human embryonic stem cells (ES cells) was developed, it has been a realistic goal to produce cells/tissues, and even organs themselves, for transplantation from ES cells, as well as from somatic stem cells, including mesenchymal stem cells (MSCs), on an industrial scale using approaches based on cell engineering/embryological engineering. In fact, for the blood, nerves, heart muscle, liver, pancreas and the like, a system for differentiation induction from ES cells is being developed, and a therapeutic technique using ES cells has already been reported.
Meanwhile, an evaluation of the toxicity of a drug to various organs is normally performed by an in vivo toxicity study by compound administration to laboratory animals such as rats; however, such studies are faulty in that (1) several days to several months are taken before onset of the toxicity, (2) a large amount of compound is required, (3) much costs are taken to purchase and maintain laboratory animals, and the like, and, in addition, the radical problem arises that the toxicity in humans is not always reflected.
To quickly predict the presence or absence of toxicity and efficiently optimize a structure in the initial stage of development of a pharmaceutical compound, it is essential to construct an in vitro screening system. In in vitro screening, it is common practice to use a primary culture of a tissue isolated from various organs or a cell line established therefrom; however, primary culture involves the above-described drawback (3) as with in vivo studies; meanwhile, when a cell line is used, problems arise, including the disappearance of the characteristics of the organ or tissue from which the cell line is derived during repeated passage, and the inability to fully reflect the pathology of the organ or tissue as an organic integration thereof on the basis of individual cells. For these reasons, use of a human cell does not always accurately reflect the toxicity in human individuals.
Therefore, there is a major demand for the utilization of tissues/organs that have developed as a result of differentiation induction from stem cells, including ES cells, for toxicity evaluations.
Confirmation of differentiation from stem cells such as ES cells into various cells is usually achieved by, in addition to morphological examination, detecting a product of a gene expressed specifically in the differentiated cell (i.e., differentiation marker gene) at a transcription level (e.g., RT-PCR, microarray analysis and the like) or a translation level (e.g., immunohistological staining and the like). For example, hepatocyte differentiation markers include the albumin gene and the like, and pancreatic P cell differentiation markers include the insulin gene and the like. However, differentiation into albumin-producing cells or insulin-producing cells, if confirmed, does not immediately lead to the conclusion that they are hepatocytes or pancreatic β cells. In fact, a report is available that ES cells were differentiated into insulin-producing cells; however, the resulting cells differed slightly from β cells.
Therefore, to determine whether or not a tissue/organ differentiated from a stem cell such as an ES cell reflects the physiological state of a desired tissue or organ, i.e., whether or not the tissue or organ is exactly the desired tissue or organ, it seems important to comprehensively analyze the expression of a large number of differentiation marker genes, rather than to rely on the expression of a single or several kinds of differentiation marker genes.
Some sets of various tissue/organ-specific genes that can serve as differentiation marker genes have been reported (for example, for human tissue-specific gene sets, see the patent document 1 and the like). However, there are a variety of definitions (criteria) for a specific gene, which have not been unified. For example, the present inventors previously announced that they extracted a crab-eating macaque tissue/organ-specific gene group from results of a comprehensive gene expression analysis in various organs using an independently developed crab-eating macaque DNA microarray (non-patent document 1), wherein a gene whose expression level in a specified tissue (organ) exceeded [the median (50% value) of the expression level in each other tissue/organ]+[75% value of the expression level in the each other tissue/organ×2] was extracted as a gene highly expressed specifically in the specified tissue (organ). This method of extraction, as a means for comprehensively identifying tissue/organ-specific genes, was better than conventionally known methods; however, even so, the specific highly expressed genes extracted by the method sometimes included those that cannot be said to be truly specific for the tissue (organ).    Patent document 1: JP-A-2004-135552    Non-patent document 1: The Journal of Toxicological Science, Vol. 31 Supplement, page S168, 2006, Title: Development of crab-eating macaque DNA microarray and search for organ-specific highly expressed genes